AltaLink, the largest electricity transmitter in Canada’s western province of Alberta, operates in a geographically diverse area. Terrain ranges from desert to plains, from foothills to towering mountains – all with a number of additional ‘micro-terrains’ sandwiched between. There are also a myriad of crisscrossing oil and gas pipelines to contend with. The diverse topography of this service area together with the northerly location results in the network experiencing winter temperatures that can drop as low as -50°C but which also climb to +40°C in summer. Moreover, strong westerly winds known as chinooks can attain speeds up to 150 km/h and have been known to raise temperatures by over 20°C in just an hour. The unpredictable weather also includes periodic hail, snow blizzards, freezing rain, thunderstorms and even the occasional tornado.
Against this backdrop of varied landscape and highly changeable climate, AltaLink is now nearing completion of what it referred to as ‘the big build’ – its largest ever expenditure on reinforcing a transmission backbone of over 12,000 km of lines and some 280 substations. This investment, amounting to more than $4 billion over a three-year period, focused on reinforcing the 500 kV, 240 kV and 138 kV systems. It followed a period of mostly stagnant spending during the 1990s and was prompted by demand spurred on by a booming natural resources sector.
In expanding and reinforcing the network, AltaLink engineers faced an array of challenges that can adversely impact performance, from conductor galloping to flashovers, from pole fires to occasional vandalism. Also, as in many other places, there have been difficulties in obtaining public approvals for new construction and these dictated how some line sections had to be designed.
INMR travelled to Calgary in 2014 to meet project engineers and supervisory staff involved in two of AltaLink’s most important recent investments – the 500 kV Heartland Transmission Project and the ±500 kV Western Alberta Transmission Line (WATL) Project.
Ask any engineer at AltaLink about the problems facing the overhead network and the reply is almost certain to include the term ‘galloping’. This seems almost a strange destiny if one considers that Alberta is Canada’s ‘wild west’ with its annual Calgary Stampede, cowboy hats and largest indoor stadium known as the Saddledome.
Tyson Harper, a Project Engineer in transmission, explained that galloping problems in Alberta tend to be concentrated during April and May when there is a critical combination of ice forming and high winds. He reported that a family of specially strengthened towers was recently developed to better cope with these stresses and to improve their ability to handle icing up to a radius of 70 mm. Specifications for related line hardware were also raised beyond typical ANSI requirements to 300 kN and even higher. Harper pointed out that while interphase spacers are installed on some lines, these do not stop galloping but rather prevent the phases from clashing during galloping conditions and thereby reduce related outages. Galloping of 240 kV and 500 kV lines has also been dealt with by increased application of self-damping wires, which are lighter than conventional conductors and where the inner core is free to move inside the outer jacket. Tighter stringing and lower structures then become possible
While Alberta is classified by the Canadian Standards Association as a light pollution area, localized contamination is another issue that has affected network performance. During winter, roads and highways are heavily laden with salt that invariably finds its way onto lines running parallel to thoroughfares. Insulation is also affected by dust from gravel roads as well as from farming practices such as turning fields and spraying fertilizer. Some lines, especially those supplying customers in the petrochemical sector, face pockets of even heavier contamination.
Transmission Engineer Shantha Samarawickrama has worked in Project Engineering Services, a department that provides expertise for the more than 100 ongoing and recently completed projects that made up AltaLink’s ‘big build’. In this role he has been tasked with working with the utility’s various EPC providers to ensure the quality of lines that are constructed. He also provided support to deal with unexpected situations such as when two lines must cross.
Samawickrama explained that while there are standards that govern each aspect of new line construction, utilizing specific contamination studies is still a relatively new practice in Alberta. For example, there is still no overall pollution map of the province that could assist selection of the most suitable insulator geometry and creepage for each region. In the past, adjusting to localized pollution has therefore been done on a more or less empirical basis.
While most of the AltaLink overhead network still relies on porcelain insulation that has dominated for many years, use of toughened glass has been growing. For example, both the new 500 kV Heartland Line and the WATL HVDC line are insulated entirely with glass. According to Sr. Transmission Lines Engineer, Dimitri Georgopoulos, historical records have shown that about 70 per cent of line outages at AltaLink are caused not by pole, hardware or conductor failure but rather by problems with insulators. “The weakest link always seems to be the insulator,” he remarked. “For this reason, one of our maintenance goals has been to keep records on all our insulators – including location, type, brand and date of manufacture. For example, he reported that many pin as well as string insulators were installed during the 1970s and 80s when there were widespread problems of poor quality porcelain that suffered from high rates of puncture and cement growth.
While AltaLink has also made use of silicone insulators, this has been mainly for line post retrofits at 69 kV and 138 kV or where there have been isolated pockets of vandalism. Silicone insulators are also being used routinely these days on slack spans entering substations since engineers have found that glass or porcelain strings tend to ‘fold up’ under lower mechanical tension. The same has applied for applications where conductors are comparatively light.
Given the localized contamination, AltaLink carries out insulator washing on selected lines. This is done mostly during the summer, either by helicopter or from the ground, and Samarawickrama explained that establishing which lines need to be washed is based on reviewing past problems. “We focus our washing on lines that have higher risk of experiencing flashovers and outages,” he said. “Affected towers are identified and field crews are dispatched to establish what triggered the flashover, whether birds, pollution or vandalism.”
Pole top and cross-arm fires are an example of an ongoing problem linked directly to localized contamination combined with poorly performing old porcelain insulation. Project Engineer, Jason Dwyer, reported that this mostly affects lines running near highways and the underlying cause is often the same – heavy wet snow that sticks to insulators and becomes the critical wetting event. This leads to tracking but does not usually trip the line until a pole fire has erupted. Glass insulators are now being installed in place of porcelain to reduce prevalence of this problem since they are easier to inspect.
Among the most important of the new construction projects at AltaLink has been the 500 kV Heartland Line. Energized at the end of December 2013, it runs some 65 km north from the Ellerslie Substation in Edmonton to an entirely new substation called Heartland. From here, part of its projected 6000 MW capacity will be transmitted along 240 kV lines to refineries in Fort McMurray – center of Canada’s oil sands development. The Heartland Transmission Project also includes a 54-structure section of 240 kV double circuit lines, energized in November 2013.
Kevin Munroe is an AltaLink field inspector who has monitored the quality of work being performed by sub-contractors who build new lines or carry out retrofits to the network. Looking back at the experience with Heartland, he explained that among the biggest hurdles was managing the crossing of over 50 different pipelines along the line’s winding route. Other obstacles were pre-existing 138 kV and 240 kV lines that had to be passed and where a range of additional engineering challenges had to be met. For example, at one crossing near a highway interchange there was concern that induced currents from the new 500 kV line might dry out the wood on 240 kV poles located directly below and increase the risk of fire. The solution was to go instead with either fiberglass or steel structures and, because of lead-time issues, the former was eventually adopted.
To meet aesthetic concerns with low visual impact towers and facilitate obtaining public approvals, a whole new family of single pole structures has been developed. Project Engineer Dwyer reports that this work involved extensive mechanical testing by the foreign-based supplier and that he has been satisfied with the end result. According to Samarawickrama, AltaLink is now studying expanded application of other families of such non-lattice towers to help in construction of new lines in similar situations
Munroe noted that construction at AltaLink is done year-around, even during winter when wind chills send temperatures plummeting and powerful wind gusts can ground helicopters. To facilitate site access under such conditions, special mats made of rows of linked wooden poles allow passage of incoming heavy equipment to clear snow or to erect towers as well as to mount insulators and line hardware. Conductor stringing is assisted by helicopter.
AltaLink’s new ±500 kV WATL line provides a good example of construction proceeding even under difficult wintery conditions. This project is insulated with 34 standard profile bells per string for each pole and 7 on the neutral. Prior to arrival of the helicopters that did the lifting, crews assembled the glass discs on tarpaulins to avoid contact with the ground. Each assembled string was then wrapped to protect it from construction dust until ready to be lifted, at which point the wrapping was removed. Insulators arriving and stored at the site were also factory-packed in closed boxes to prevent contaminants from being deposited during transport. Said Munroe, “the key is that when we put the insulator string up it must be completely clean and free of road salt that may have fallen onto it and which might lead to tracking from the start. Washing is relatively expensive, so we want to be sure that every new line starts out with the insulators perfectly clean.”
Corona is another concern from the standpoints of noise as well as the risk of damage to silicone insulators that are now routinely used on slack spans at substation entrances. Munroe reported that every structure on WATL is equipped with corona rings up to three meters in diameter, depending on type, and which have been specially designed and tested by the supplier in Europe. Moreover, all line hardware, including conductor saddles and spacers, was required to be certified corona-free in terms of smooth surfaces with no rough edges. Even a protruding manufacturer name sticker has been known to cause noise.
Munroe explained that this corona-free requirement applied to hardware used on the 500 kV Heartland Line as well and is now even being demanded for new 240 kV lines. He also stated that AltaLink’s standard is that all hardware, including the armor rods that help damping and prevent fatigue failures, must be tested for cold weather shock loading. This is done to ensure that the steel always remains ductile, even at service temperatures down to -50°C.
From a maintenance perspective, Munroe noted that it would be normal to inspect a new transmission line such as WATL annually, using various types of patrols. He also explained that additional corona and infrared cameras have recently been purchased to assist in helicopter-based inspections, looking for defects such as nicks in wires or sharp edges that might create corona.
The various remedial measures to deal with past problems as well as all the investment recently made in new lines have already had a large impact on performance of AltaLink’s overhead network. “There’s no question that our system has improved a great deal compared to 15 years ago,” observes Munroe, whose view is only made more credible by his many years in the business. “AltaLink now has strong reliability,” added Dwyer, “and we feel that our network performance is impressive, especially if you consider the age of our system as well as the varied geography and difficult climate where it operates.”